It will be remembered that a conventional video image is constituted, in respect of each image, by two fields, known as an even-numbered field and an odd-numbered field, which are interlaced on alternate lines. At display, these fields are scanned successively in time on the screen, typically a cathode ray tube, with the lines of the second field of an image being scanned on the spaces left between the scanning lines of the first field. Relative to a progressive scanning, where the successive lines of a complete image are displayed sequentially, interlaced frame scanning makes it possible, on the one hand to double the vertical resolution while retaining the same pass band or, at equal vertical resolution to double the frame return frequency and thereby to reduce the effects of flickering.
Analog and digital video signals are generally formatted in the form of interlaced frames, known as “interlaced video”. Systems need therefore to be provided that convert interlaced video signals into non-interlaced video when it is necessary to display them in progressive scanning mode. The latter is used particularly in addressable line-by-line display devices such as plasma panels, liquid crystal displays, organic light-emitting diodes, etc. that use an electrode matrix network for example.
Systems of this kind, known as “de-interlacing” systems, must produce all the displayed lines of an image from only one field of the two fields of which it consists. In fact, since a field contains only one line in two of the image, de-interlacing calls on interpolation techniques that allow the content of the missing lines to be determined on the basis of the content of their adjacent lines and where appropriate the adjacent fields.
Conventionally, the interpolated output of the de-interlaced signal is constituted from a spatial interpolation and a temporal interpolation according to a variable weighting. The contribution of each of these two interpolations in the composition is generally variable over a range including the two extreme situations, namely 100% spatial interpolation and 100% temporal interpolation respectively. The decision in question therefore consists in determining the respective spatial and temporal interpolation weightings in the constitution of the interpolated output signal. This determination is made by a mixer, more generally known as a “fader”, which receives the two types of interpolation at input and produces the interpolated signal at output with the aforementioned weightings.
In basic interlacing, the missing lines are in most cases interpolated by low-pass spatial filtering of the existing adjacent lines. A field fusion technique is used when the medium is a film, since it is then known that an odd-numbered field followed by the corresponding even-numbered field are connected at the same temporal position or that an even-numbered field followed by the corresponding odd-numbered field are connected at the same temporal position, the film detection systems allowing one or other of the situations to be identified. In a more general way, temporal interpolation may be used in the case of a fixed image. This gives the advantage of avoiding unnecessary losses of definition caused by low-pass spatial filtering.
In most de-interlacing systems that use both temporal interpolation and spatial interpolation, a decision is made once per field, generally taking the safest option, based on a film detection.
This implies that time domain filtering is used only if the whole image is detected as being static. This often leads to switching artifacts, where the image has a high level of sharpness when everything is motionless, but loses it as soon as motion appears.
Furthermore, we are moving towards video contents that have static parts inserted into them, such as logos in moving images or scrolling text over a quasi-static image. In this case, it is desirable to use an interpolation that is more spatial in some parts of the image and an interpolation that is more temporal in others. There are also sources where some parts of the image have a video content whereas other parts come from a film.
The result is that in all de-interlacing systems requiring a film detection, the de-interlacing is not optimum for the whole image.
Sophisticated motion compensation de-interlacing systems also suffer from this last limitation.
Accordingly, a need exists for a low-cost de-interlacing system that overcomes the limitations of the prior art and which does not require a high level of performance at medium detection system level, whatever the medium may be (film, video, or a mixture of the two)